This reverts commit a495f1a67f.
This ended up not being an issue. What we _do_ have to do, sadly, is to
have a mutual recursion context stack per plugin. Otherwise multiple
plugin instances can deadlock eachother.
FabFilter plugins will exchange messages that have to be handled from
the GUI thread, or they'll get stuck waiting on a synchronisation
object. This probably hurts GUI performance significantly but luckily it
only affects Ardour.
Instead of serializing the actual `YaMessage`, for the reasons mentioned
in the comments. This was needed to stop iZotope VocalSynth 2 in Ardour
from segfaulting when editing parameters, because that plugin is
apparently being very naughty.